The present invention relates generally to the field of microwave-assisted chemistry techniques, and in particular relates to instrumentation and techniques for conducting loss-on-drying analyses and calculations for a wide variety of materials.
Measuring the sample volatile content (which in many cases is the moisture content) is a frequent and repetitive chore in many analytical laboratories. For example, in a manufacturing setting, the measurement of sample volatile content may be an important step in a quality control procedure. If the time for conducting the analysis is long, then poor quality samples may not be detected for several hours or days. In this circumstance, the manufacturing facility may have continued producing the lower quality product throughout the time necessary for conducting the test. Accordingly, a large quantity of poor quality material may have been produced before the quality problem was discovered. Such a delay often leads to cost overruns and manufacturing delays, as the poor quality product may require disposal and the manufacturing process must begin again.
In its simplest form, determining volatile or moisture content consists of weighing a representative sample of material, drying the material, then re-weighing the material to ascertain the losses on drying and, consequently, the initial volatile content of the sample. Convective, hot-air ovens, which are often used for this task, can be relatively slow to bring the sample to “oven-dry” equilibrium. Such devices can also be expensive to operate as they inefficiently consume energy. These problems lessen the utility of hot-air devices for volatile analysis.
Drying certain substances using microwave energy to determine volatile or moisture content is generally convenient and precise. The term “microwaves” refers to that portion of the electromagnetic spectrum between about 300 and 300,000 megahertz (MHz) with wavelengths of between about one millimeter (1 mm) and one meter (1 m). These are, of course, arbitrary boundaries, but help quantify microwaves as falling below the frequencies of infrared (IR) radiation and above those referred to as radio frequencies. Similarly, given the well-established inverse relationship between frequency and wavelength, microwaves have longer wavelengths than infrared radiation, but shorter than radio frequency wavelengths. Additionally, a microwave instrument incorporating a micro-processor can monitor the drying curve (weight loss vs. time) of a sample and can predict the final dried weight (and thus the original moisture content) based on an initial portion of the drying curve. Such analyses may be conducted in about two to three minutes for samples that contain free water.
More importantly, microwave drying to measure moisture content is usually faster than equivalent hot-air methods. As in hot-air techniques, however, certain substances tend to burn, rather than merely become dry, when microwave power is applied to them. Stated differently, the rapid manner in which microwaves tend to interact with certain materials, which is an obvious advantage in some circumstances, can cause secondary heating of other materials that is disadvantageous (at lest for volatile or moisture measurement purposes). Certain food products such as cheese are exemplary (although certainly not limiting) of materials that tend to burn rather than dry when exposed to microwaves.
Additionally, microwaves interact with materials in a fashion known as “coupling,” i.e., the response of the materials (“the load”) to the microwave radiation. Some materials do not couple well with microwave energy, making drying or other volatile removal techniques difficult or imprecise. Other materials couple well when their moisture content, or content of other microwave-responsive materials (e.g., alcohols and other polar solvents), is high. As they dry under the influence of microwaves, however, they couple less and less effectively; i.e., the load changes. As a result, the effect of the microwaves on the sample becomes less satisfactory and more difficult to control. In turn, the sample can tend to burn rather than dry, or degrade in some other undesired fashion. Both circumstances, of course, tend to produce unsatisfactory results.
As another factor, volatiles, such as “loose” water (i.e., not bound to any compound or crystal) respond quickly to microwave radiation, but “bound” water (i.e., water of hydration in compounds such as sodium carbonate monohydrate, Na2CO3.H2O) and nonpolar volatiles (e.g., low molecular weight hydrocarbons and related compounds) are typically unresponsive to microwave radiation. Instead, such bound water or other volatiles must be driven off thermally; i.e., by heat conducted from the surroundings.
Thus, microwaves can help remove bound water from a sample when the sample contains other materials that are responsive to microwaves. In such cases, the secondary heat generated in (or by) the microwave-responsive materials can help release bound water. The nature of microwave radiation is such, however, that not all such materials or surroundings may be heated when exposed to microwaves. Thus, loss-on-drying measurements using microwaves are typically less satisfactory for determining bound water than are more conventional heating methods.
In order to take advantage of the speed of microwave coupling for samples that do not readily absorb or couple with microwaves, techniques have been incorporated in which a sample is placed on a material that absorbs microwaves and becomes heated in response to those microwaves (often referred to as a susceptor). U.S. Pat. No. 4,681,996 is an example of one such technique. As set forth therein, the goal is for the thermally-responsive material to conductively heat the sample to release the bound water. Theoretically, a truly synergistic effect should be obtained because the thermally heated material heats the sample to remove bound water while the free water responds to, and is removed by, the direct effect of the microwaves.
In such susceptor techniques, when non-polar solvents are present with bound or free water in material to be analyzed for volatiles, they are likewise volatilized by the thermal heat generated by the susceptor, while the free water (which may have been thermally released from a bound form), is vaporized by the microwave radiation. Thus, volatiles may be quickly removed from the sample whether the volatiles are bound water, free water, other polar materials, or non-polar compounds.
Susceptor techniques, however, are less successful in actual practice. As one disadvantage, the necessary susceptors are often self-limiting in temperature response to microwaves, and thus different compositions are required to obtain different desired temperatures.
As a third disadvantage, the predictability of a susceptor's temperature response can be erratic. As known to those familiar with content analysis, certain standardized drying tests are based upon heating a sample to, and maintaining the sample at, a specified temperature for a specified time. The weight loss under such conditions provides useful and desired information, provided the test is run under the specified conditions. Thus, absent such temperature control, microwave techniques may be less attractive for such standardized protocols.
As another disadvantage, the susceptor may tend to heat the sample unevenly. For example, in many circumstances, the portion of the sample in direct contact with the susceptor may become warmer than portions of the sample that are more remote. Such uneven temperatures may lead to incomplete removal of bound moisture as well as inaccurate loss-on-drying analyses.
Bound water may be removed in some circumstances by applying infrared radiation to a sample. Infrared radiation succeeds in driving off bound water (as well as any free water) by raising the temperature of the sample to an extent that overcomes the activation energy of the water-molecule bond. Infrared drying is also faster than oven drying for many samples. Nevertheless, infrared radiation tends to heat moisture-containing samples relatively slowly as compared to microwaves. Furthermore, infrared radiation does not couple with materials. Instead it typically heats the surface (or near surface) of the material following which the heat conducts inwardly; and typically takes time to do so. Infrared radiation will, however, heat almost all materials to some extent, and thus it offers advantages for materials that do not couple with microwaves.
Merely using two devices (e.g., one microwave and one infrared) to remove the two types of volatiles does not provide a satisfactory solution to the problem because moving the sample between devices typically results in at least some cooling, some loss of time (efficiency), the potential to regain moisture (under principles of physical and chemical equilibrium), and an increase in the experimental uncertainty (accuracy and precision) of the resulting measurement. Furthermore, if a sample is moved from a first balance in a microwave cavity to a second (separate) balance exposed to infrared radiation, the tare on the first balance would be meaningless with respect to the use of the second balance.
Accordingly, a need exists for loss-on-drying instrumentation and techniques that minimize or eliminate the disadvantages of prior methods or devices with respect to a wider variety of sample materials.